Active chilled beams provide an energy-efficient way to provide sensible cooling to a space. High energy efficiency can be achieved by accomplishing most of the space sensible cooling utilizing moderate temperature chilled water while minimizing the airflow ducted to the space. In a number of embodiments, the outdoor ventilation airflow is the only blown air used to provide all of the cooling and heating energy to the space. Typically, this airflow may be only 25%-35% of that used by conventional cooling systems (i.e., VAV or fan coil systems) thereby saving significant fan energy. Active chilled beams can deliver this relatively small outdoor or primary airflow though slots or nozzles within the beam to cause induction of room air through the integrated coil. In a typical application, this “induced room air” may be 3-4 times the primary airflow volume, so the final airflow volume delivered to the room may be similar to that delivered by convention cooling systems, but only a fraction of the fan horsepower may be used. Excellent indoor air quality can be achieved, in various embodiments, using active chilled beams since outdoor air is ducted directly to the individual zones and is provided continuously. In certain embodiments, active chilled beams also provide the benefit of very low noise generation, making them well suited to meet the more stringent sound criteria recently incorporated into building codes for applications such as school classrooms. They may also benefit from ideal airflow distribution and eliminate drafts common with conventional forced air systems.
Passive beams, on the other hand, do not have air connections, and thereby do not deliver nor induce airflow. They incorporate a chilled coil or plate and rely on natural convection and radiant heat transfer to condition the space. They typically work with a reverse chimney effect, meaning that the cooler air near the beam's chilled surface has a higher density than the surrounding air and therefore the cool air flows downward to the occupied space. A common feature of typical active and passive chilled beams that both cool and heat is that they require chilled or hot water to be passed through the device to function, involve a significant amount of costly chilled and hot water piping, require careful control over the chilled water temperature, and air flow serving the beams and the space served by the beams must be effectively dehumidified to avoid condensation.
In a typical chilled-beam system, very cold water is created by the chiller typically having a temperature of about 45 degree F. The very cold water in the “primary chilled water loop” is delivered directly to the primary air handling system that produces the primary airflow that is delivered to the active chilled beams, often referred to as a dedicated outdoor air system (DOAS). This primary air system typically requires this very cold water in order to dehumidify the primary air delivered to the beams to the level appropriate to handle all of the space latent load (humidity) associated with the occupants, infiltration and other moisture sources. Lower than normal supply dew point air is required since these internal latent loads are accommodated using the relatively low primary airflow volume at each zone. Effective space humidity control can be important for many chilled-beam system applications to avoid condensation on the coils since they are most commonly designed to be 100% sensible-only devices.
In some embodiments, very cold refrigerant leaving the chiller is passed through a heat exchanger before being returned to chiller for re-cooling. A portion of the water from the secondary water loop is passed through the secondary side of the plate frame heat exchanger to create the moderate temperature chiller water required by the chilled beams. Typically, the water temperature delivered to the chilled beams will be much warmer than delivered to the DOAS system which supplies dehumidified outdoor air to the active chilled beams to avoid condensation on the coil surfaces, the chilled water pipes, control valves and other devices that are part of the chilled-beam system. Water in the range of 56 to 60 degrees F. is commonly used with 58 degrees being typical. To create and maintain this 58 degree F. water that is delivered via the secondary water loop to the chilled beams, a 3 way modulating control valve is commonly used to distribute a portion of the warmed water that has returned from the secondary water loop after leaving the chilled beams through the heat exchanger while also diverting (bypassing) the remainder of the secondary loop return water around the heat exchanger. These two streams are typically mixed before entering the secondary water loop pump.
To determine the proportions of water that goes through the heat exchanger and the portion that is bypassed, the three way modulating valve can be controlled by a temperature sensor measuring the temperature of the water leaving the secondary water loop pump. The 58 degree F. secondary chilled water can be pumped through the supply water pipe loop which carries this water at a constant temperature through all of the zones, distributing the volume of water as needed to the beams in each zone, zone after zone, until the supply water loop reaches the very last zone where the last bit of supply water is injected into the final beams. This marks the end of the supply water loop in this example.
Based on a call for cooling from the zone thermostat, in this particular example, a two-way valve can be fully opened allowing the water to pass from the secondary chilled water supply loop, through the coils contained within the chilled beams, and into the secondary chilled water return loop. In this way, the designed flow of 58 degree water is passed through the chilled beams to provide the cooling to the zone. This chilled water continues to pass through the beams at full flow, regardless of the space load, until the space control set point, plus any applicable dead band, is reached. At this point the two way control valve is closed and the water flow is stopped until there is a need for additional cooling.
In this example of a typical state-of-the-art chilled beam design, a secondary chilled water return loop pipe is installed adjacent to the secondary chilled water supply loop such that there are two distinct pipe branches (two pipe loop) run throughout the building, just for the chilled water. As with the supply loop, the water leaving the chilled beams in the last zone is injected into the secondary chilled water return loop and the volume of water continues to build until the full system flow is returned to the 3 way modulating valve to begin another circuit. The approximate 58 degree F. water entering the chilled beams in the various zones picks up heat energy as it cools the individual zones as a result of the relatively warm room air (typically 76 degrees) passes over the coils contained within the active chilled beams throughout the building. As a result, the secondary chilled water return loop water temperature returning back to the 3 way modulating valve and water loop pump is typically warmed to a temperature of about 64 degree F.
Although some chilled-beam systems provide cooling only, various chilled beams, both active and passive, can provide heating as well as cooling. When heating is required, the current state-of-the-art design uses a coil that has “4 pipes” rather than two as described for cooling-only applications. The coil has a cold water inlet and a cold water outlet in addition to a hot water inlet and a hot water outlet (i.e., 4 pipes). Typically, an eight-pass coil, that would be used in a cooling-only beam to provide the maximum cooling output, is modified to allocate six passes for cooling and two passes for heating. This results in a significant reduction in potential coil cooling power (typical reduction of about 15%-20%) while providing adequate heating capacity in most cases, since the required heating energy (BTUs) is most often considerably less than the cooling capacity needed. This is logical since the sensible heating load provided by the people and lighting is provided to the space whether in cooling or heating mode (i.e., a heating credit).
When heating is added, another heat exchanger can be added as part of the boiler system to condition the warm water (typically in the range of 100 degrees F.) to the beams. Typical heating loop water temperatures (say 140 degrees F.) should not be provided to the beams when in heating mode, in many applications, since the low velocity air leaving the beams can result in stratification which compromises both comfort of the occupants and the heating efficiency of the coils in the heating beams. Consequently, another separate secondary heating water loop (supply and return) in addition to that required for the cooling loop, is typically required for the beam distribution system, involving a duplication of pipe, control valves, 3-way valve, and pump, as examples. In addition, controls and power need to be connected to all valves, and pipes must be insulated and balanced for both the entire cooling and heating portion of the beam system.
While effective, there are a number of limitations and problems with the current state-of-the-art chilled-beam system design. Some of these limitations are considered major barriers by many engineering design firms, causing them to continue the use less energy efficient conventional HVAC systems. First, the current state-of-the-art solution requires two separate chilled water loops—one for the chilled beams and one for the DOAS system delivering the air to the chilled beams. This is due to the water temperature required by each system. To accomplish the dehumidification required by the outdoor/primary airflow to the beams, a low supply air dew point in the range of 45 to 50 degrees F. is required. As a result, the water temperature delivered to the coil within the DOAS has to be in the range of 40 to 45 degrees, depending upon the type of DOAS used and the project space latent loads. As previously mentioned, to avoid condensation on the beams and to optimize cooling comfort (avoid dumping of cool air and drafts), the water temperature delivered to the chilled beams typically needs to be in the range of 56 to 60 degrees F. A similar situation exists for the hot water loops. The DOAS and other hot water needs may require a much hotter water temperature than desired for optimum performance of the beams. This duplication of water loops and associated cost has proven to be a significant barrier to acceptance and use of chilled-beam technology. As a result, it would be beneficial if only one water loop was required for both the DOAS and the chilled beam network.
Second, in many applications, the greatest incremental cost of a chilled-beam system is the material and installation cost associated with the water piping. Since the current state-of-the-art chilled-beam system design involves both a supply and return piping network throughout the building for each of the hot and cold water lines, and these four runs of distribution piping commonly are copper, the cost is considerable. Adding to the problem of high cost associated with the current approach, the size/diameter of the pipe must be relatively large to accommodate the high water flows associated with the moderate chilled and hot water temperatures required by the beams. For example, the water entering the chilled beam at say 58 degrees F. and leaving at 64 degree F. (6 degree delta temperature) requires three times the water flow to accomplish the same cooling power as a system designed to deliver water at 46 degrees and leaving at the same 64 degree temperature (18 degree delta T). Putting this in terms of pipe size, a pipe having the diameter of 2″ delivering chilled water at 46 degrees would have to be increased to approximately 3.5″ in diameter.
The difference in the cost of the pipe, connectors, valves and all other components and associated labor needed to accommodate this increase in pipe size over that typically used by more conventional technologies is much higher than what many design engineering firms and/or owners are willing to invest. A similar increase in pipe size is associated with the need to use 100 degree water, for example, for heating vs. typical hot water loop temperatures in the range of 140 degrees. This high cost of chilled and hot water piping has proven to be a barrier to acceptance and use of chilled-beam technology. As a result, it would be beneficial, in a number of applications, if fewer pipe loops, pipe having a smaller size, or both, could be employed.
Third, since water must be pumped at a relatively-high flow rate (due to the moderate delta T discussed previously), through both the supply and return water distribution pipe networks, for both cooling and heating, plus the zone piping to the beams, the coils and series of valves, the pumping energy can be relatively high. Since the current state-of-the-art chilled beam design utilizes an on/off control valve, the flow through the beams is constant and capacity control (when the spaces need less heating or cooling) is accomplished by cycling the water to the beams on and off. So, at peak cooling, all of the beams are delivered the full water flow and the main pump must provide this high pressure at the full flow.
Further, the use of a single pump to provide water to all zones can be both limiting and problematic. For example, the pump must provide as much static pressure as is required by the last zones on the system (those furthest from the pump). If this zone has, for example, more sensible loads than other zones (e.g., top floor with more windows) then the scheduled water flows for these beams, and thereby the water pressure loss through the coils, will be high. To overcome this pressure loss and drive the water through the coils, the main pump pressure must be increased for the entire system requiring a significant increase in energy as a result. Another common problem is that the installation of the piping and valves, due to jobsite limitations, is often less than ideal (e.g., more bends and turns than the original design) which adds pressure loss to the system which must be overcome by the main pump. Likewise, should the loads be under-estimated in a zone or if the use of the zone changes (e.g., an overcrowded school moves more children into a classroom than design) more cooling will be required. The main pump may not have the capacity to increase the pressure through the entire system to accommodate the peak load in a problematic zone or zones that need additional cooling.
Another challenge is that much of the pressure loss within the main chilled beam distribution piping can occur between the main supply water distribution pipe and the main return water pipe. This includes the chilled beams, the valves, and the piping connected to the beams. In many cases, the pipe connecting the beams to the main water lines is done in flexible PEX type tubing using special connectors that reduce installation labor but often increase the pressure loss through the system. Yet another limitation is that the water flow to each zone has to be measured and balanced so that the chilled beams get the design flow of water in both the heating and cooling modes. This is commonly done at or near the two-way control valve previously mentioned. Often this is accomplished by adding restriction to control flow or using a flow regulation valve rated for the water flows desired. In both cases, the devices set the flow at a fixed water pressure provided by the main circulation pump, and in the case of the flow regulation valve, ensuring that the flow does not exceed the design value. In cases where the system efficiency could benefit from a variation, however, either up or down, of the water flow to the beams, for either efficiency reasons or capacity boost, this can not be accomplished with the prior art design approach. For all of these reasons, it would be beneficial to provide localized pumping at each zone to provide added capacity or pressure as needed or to benefit from lower pressure losses, for example with reduced flow, for energy efficiency reasons. This concern regarding how to increase the heating or cooling capacity at the zone at the end of the piping system has proven to be a barrier to acceptance and use of chilled-beam technology.
Fourth, perhaps the most significant barrier to acceptance of the chilled-beam technology in moderately or severely hot and humid climates, commonplace in the US and Asia, is the concern for condensation on the beams. Most of the higher performing chilled beam products are designed to have the coils within the beams operate as sensible-only devices (i.e., no moisture removal) so that they can be installed throughout the occupied space without the installation of a drain pan and eliminating the high cost of condensation collection piping. While there are many advantages to operating chilled beams as sensible-only devices, should condensation occur, allowing water to drip directly into the occupied space, it would be a very serious problem in most applications and is typically unacceptable.
The primary line of defense for prohibiting condensation at the beams is to provide enough primary air, at a low enough humidity level, so that the space dew point is always maintained below the water temperature entering the beams during the cooling mode. With proper engineering design, load estimates, and effective DOAS equipment, this can be accomplished. Design errors can occur, however. Also, not all possible condensing scenarios can be avoided in this fashion. For example, if a door or window to a space served by the chilled beams is allowed to be open during a humid day, the space dew point can rise above the design point despite the delivery of the design quantity of dehumidified primary air.
Another common scenario is when a room is occupied with many more people than was used to determine the design primary airflow quantity and dew point. An over-crowed classroom or meeting room are two good examples of this occurrence. A third and very common scenario where the space humidity could rise to the point of causing beam condensation is during times of extreme outdoor heat and/or humidity. If the DOAS is sized to deliver air at a certain dew point at a moderate design condition, and this condition is exceeded, or if the condenser side efficiency of the chiller system is impacted, or the chilled water temperature rises slightly—all of which are common—the supply air dew point of the primary air delivered to the space by the chilled beams will increase. In all these cases, condensation could occur.
A prior chilled-beam system design addresses this issue by installing a condensation (moisture) sensor on the surface of the chilled water pipe serving the chilled beams in each zone. If the dew point is high enough to cause condensation at the monitored point, the liquid water creates a circuit sending a signal confirming condensation which is then used to close the control valve serving all beams in the zone. While, when working properly, this approach can provide a level of protection against dripping water from the beams into the occupied space, it immediately cuts all cooling provided by the chilled beams to the occupied space, which is often not acceptable to the users of the space nor considered an acceptable solution by many design engineers. In many of the scenarios mentioned above—meeting room, over-crowded classroom, and an open door for a short period of time—it is desired that cooling still be provided to the space despite a modest rise in space dew point. For all of these reasons, it would be highly beneficial, in many applications, to provide an active condensation control system for chilled beams that can respond to limit the risk of condensation while simultaneously providing effective cooling to the occupied space. This concern regarding condensation on the beams and how to avoid eliminating cooling in response to a condensation signal has proven to be a barrier to acceptance and use of chilled-beam technology.
As described previously, when a state-of-the-art chilled-beam system was designed to provide both heating and cooling, the circuiting of the coils within the beam were modified to reduce the number of cooling passes to allow for heating passes. In climates and buildings where there is a modest heating load, it is common to change a coil that would have, for example, 8 total passes, to provide 6 passes for cooling and 2 passes for heating. In colder climates, however, it is not uncommon for the coil to be changed so that 4 passes are used for heating and 4 passes are used for cooling. Increasing the number of cooling passes from 6 to 8 improves the cooling power output (BTUs) from the coil by approximately 15-20%. Increasing the number of passes from 4 to 8 improves the coil output by up to 30% at typical design conditions. Therefore, when coil passes are allocated for heating and the number of cooling passes are decreased, the amount of cooling that can be delivered by the chilled beam at a given design point (e.g., primary airflow, water temperature, water flow rate) is substantially reduced. There are few viable options to make up for this loss of performance. The primary airflow can be increased to provide more cooling associated with the air delivered to the room, but this is a costly solution since it involves both fan energy and more conditioning at the DOAS. Lowering the water temperature would provide added cooling output, but doing so increases the risk of condensation at the beams and, with the state-of-the-art design, means that this lower water temperature is provided to all zones. The colder water temperature to the beams would require drier air from the DOAS which also increases energy consumption.
The most viable option with the prior art design to compensate for the reduced beam capacity associated with fewer cooling passes may be to both increase the water flow to the beam and increase the length of the beam. Increasing the water flow enough to improve performance in the amount appropriate to counter the loss associated with reduced cooling passes, however, has a significant impact on the energy consumed by the main system pump. Increasing the beam length is the best option with regard to energy efficiency, but the cost of each beam would be increased by 15% to 25% and there is a practical limit to how much ceiling area can be allocated for the beams since light fixtures typically must also be effectively accommodated. In addition to the higher cost associated with increasing the length of the beam, there is also a significant cost associated with changing the coil to allow for both heating and cooling. In fact, increasing the length of a chilled beam by 25% and adding both heating and cooling capacity to the coil would typically double the cost of the chilled beam when compared to a beam where all passes could be used for both cooling and heating. For all of these reasons, it would be highly beneficial to have a system that allows all passes of the coil within the chilled beam to be utilized for either heating or cooling, since it would result in the use of fewer or shorter beams, at a lower cost, to provide the equivalent amount of cooling/heating output as longer 4 or 6 pipe beams.
Further, the current state-of-the-art chilled-beam system layout (as described) employs a constant flow volume of water to the beams maintained at a constant temperature, and the only method of control is to turn the flow on or off. Therefore, full cooling or heating capacity is provided as the control valve opens and closes in response to a space temperature sensor. As a result, very little flexibility is provided to accommodate varying load conditions. For example, should a room experience a heat gain that is greater than design due to increased occupancy, higher than anticipated solar load or degradation to the chilled or hot water temperature, there is no way for the system to respond. Once opened, the maximum cooling or heating capacity is recognized and there is no way to deliver more.
Conversely, when the room is at part load conditions, where occupancy is low or when the solar load is reduced (e.g., cloudy day) the only way to reduce the cooling load is to repeatedly cycle the flow to the beams on and off. While this addresses the lower cooling requirement, it does so in a way that does not efficiently use pump energy and there can be more frequent than desired swings in room temperature. There have also been complaints of nuisance noise associated with the control valves turning on and off associated with the initial in-rush of high pressure water. Since chilled beams are otherwise a very quite technology, this noise is easily detected and is not easily remedied.
During heating, when the zones are occupied and lights are on, the amount of heating required relative to the cooling energy (BTUs) needed at peak conditions is relatively low. As a result, the state-of-the-art beam design for the heating system is typically based upon a much lower water flow to the beams in an attempt to save piping cost (lower flow smaller diameter pipe) and to match the beam capacity to the occupied room load. This can be problematic, however, if the control system uses a night setback temperature that requires a rapid morning warm up mode (i.e., higher heating output on a temporary basis). A similar problem exists during unusually cold days when the envelope heat losses from the building are greater than design. For all of these reasons, it would be highly beneficial to have a chilled beam water distribution and control system that could respond to extreme cooling or heating load conditions by providing a boost mode to increase the output from the beams. It would also be highly beneficial to have a chilled beam water distribution and control system that could respond to part load and low load conditions in a more energy efficient manner and avoid the nuisance noise associated with the repeated opening and closing of the on/off control valve used by the current approach.
Even further, for optimizing energy efficiency, there is a strong desire to reduce the amount of outdoor/primary airflow delivered to the building spaces via the chilled beams during times of low occupancy or no occupancy. Going back to a typical school example, most weekends, evenings and summer months, the school remains mostly unoccupied. In such cases, very little ventilation air is required—potentially, only that needed for building pressurization to avoid high humidity infiltration loads. Likewise, since the building is unoccupied, the amount of heat normally generated by the lights and people is removed from the space, so only a small fraction of the peak cooling output from the beams is required. Some cooling may still be required, however, to maintain minimum setback conditions. In addition, there are many reasons why certain rooms might be in normal use during any of the common unoccupied periods cited, and the system may need to respond to the individual cooling and heating needs of these spaces.
Active chilled beams require a minimum amount of air for them to function effectively. As importantly, in a number of applications, the primary airflow is the only viable source of space dehumidification and adequate supply must be provided at all times for this purpose. Therefore, the primary airflow typically should not be turned off completely, in a number of applications, but it can typically be reduced, for example, by approximately 50-60%. At these levels, the desired cooling capacity can typically still be provided, since the zone sensible load is greatly reduced during unoccupied periods, with significant fan energy savings being recognized. For example, cutting the supply and return airflows to a 5,000 cfm DOAS operating at a total static pressure per airstream of 4″ by 60% reduces the fan electrical energy by more than 90% (6.25 KW vs. 0.4 KW).
While the potential energy savings are significant, the VAV enhancement presents serious challenges to the current chilled-beam system design approach. As mentioned, if the airflow reduction is too low to handle the space latent load, the space dew point may climb causing condensation on the beams. The beam condensation sensor may detect this occurrence, and shut off the chilled water to the beam. As a result, the rooms could remain without cooling for extended periods making it difficult to cool them back down in a timely manner, for example, the next morning.
VAV can also be tied to occupancy or CO2 sensors, for example, to allow the airflow to be reduced to the chilled beams when there is only partial occupancy—for example, a teacher in a room grading papers. In this case, the lights would still be on adding sensible load to the space and there can still be a significant sensible solar load to the classroom on a sunny day. At times like this, there may not be adequate cooling capacity delivered by the beams. When the airflow is reduced to the chilled beam, the induction air (air passed through the coil) is significantly reduced. At the same time, the cooling provided by the primary air is also reduced. If the room gets too hot, there is no way for the prior art design to respond. It would therefore be highly beneficial to have a system for controlling chilled-beam systems that can better respond to the challenges associated with a VAV application; being able to actively avoid beam condensation if the dehumidification provided by the primary air is inadequate and providing a boost to cooling capacity from the beams at the low primary airflow conditions.
Since, as previously discussed, the state-of-the-art chilled-beam system design uses supply chilled water having a temperature of approximately 58 degrees F., and since the water temperature leaving the beams is generally in the range of 65 degrees F., the delta T across the system is approximately 7 degrees F. A well designed system may use a variable speed primary water pump to respond to part load cooling and heating conditions while maintaining a constant pressure within the supply water distribution pipe network. As a result, as the load on the chilled beams is reduced, the two-way valves are cycled taking less water from the supply loop, and the pump reduces flow to save energy. Although chilled water flow is reduced, the temperature differential (delta T) across the secondary heat exchanger or chiller remains low (e.g., only about 7 degrees) which impacts negatively on chiller performance. As a result, it would be highly beneficial to have a system for controlling chilled-beam systems that can be operated to provide a greater delta T across the chiller or heat exchanger to increase chiller performance.
Moreover, the typical state-of-the-art chilled beam design system is independent from the DOAS/primary air system that delivers primary airflow to the beam system. The temperature sensor assigned to each zone monitors the sensible cooling needs of the zone but provides no feedback to the DOAS to provide guidance as to the dew point appropriate to satisfy the space latent load. Nor does it allow for optimization of the overall system. This prior art example relies solely on the load calculations made regarding space latent loads, perhaps adjusting the supply air dew point from the DOAS/primary air system based on outdoor air dew point or the relative humidity of the air returning to the DOAS/primary air system from the mixture of all zones. For the many reasons discussed up to this point regarding limitations of the state-of-the-art chilled-beam system design, it would be highly beneficial to allow for active communication of the real-time conditions in each zone (e.g., zone air temperature and humidity, supply water temperature, occupancy, etc.) to the DOAS/primary air system (and or building BAS) so that more effective system performance and condensation control strategies could be implemented.
Other needs or potential for benefit or improvement may also be described herein or known in the HVAC or control industries. Room for improvement exists over the prior art in these and other areas that may be apparent to a person of ordinary skill in the art having studied this document.
These drawings illustrate, among other things, examples of certain components and aspects of particular embodiments of the invention. Other embodiments may differ. Various embodiments may include some or all of the components or aspects shown in the drawings, described in the specification, shown or described in other documents that are incorporated by reference, known in the art, or a combination thereof, as examples. The drawings herein are of a schematic nature and are not necessarily drawn to scale. Further, embodiments of the invention can include a subcombination of the components shown in any particular drawing, components from multiple drawings, or both.